This invention relates generally to imaging systems, and more particularly to methods and apparatus for producing images of objects embedded in diffusing media, specifically including unstable diffusing media, such as the flesh of a living being.
The problems associated with imaging through a diffusing, or irregular, medium are among the most challenging of imaging science. For example, the medium may be a turbulent atmosphere, through which the object to be imaged is recognizable, but the image thereof is degraded, or at the other extreme, the medium may be so severely scattering that, although light is transmitted therethrough, the wavefronts are distorted beyond recognition. Such materials are sometimes called "translucent." If light from an object is transmitted through such a medium, the image that can be formed from the emerging light is extremely coarse, or perhaps no image whatsoever of the object can be seen.
When a medium transmits light, irrespective of the extent to which it is distorted, practitioners in this optical art have sought ways to recover the information that had been impressed on the light. The problem, however, is extremely difficult and has, for the most part, remained intractable.
One of the most important examples of imaging through scattering media lies in the field of medical imaging. The possibility of seeing absorbing structures inside living tissue, without resort to invasion, has attracted many researchers. However, this form of medical imaging presents problems which are particularly challenging because living tissue is extremely diffusing, unstable, and highly absorbing at many wavelengths.
A short pulse of light which passes through a highly scattering medium undergoes multiple scatter events and emerges as a greatly elongated pulse. The light which is scattered the least has travelled the shortest path, and therefore, emerges first. Nevertheless, a shadowgraph image projected onto the emerging surface will in general be extremely blurred. The originally short pulse emerges as a pulse orders of magnitude longer in duration, as a result of the multiple scattering. The light which first emerges from the diffusing medium is therefore the least scattered component.
There is, therefore, a need to isolate the first emerging light from the remainder of the elongated emerging pulse. In order to resolve small objects, illustratively on the order of 1 or 2 mm, subpicosecond light pulses would be required. In addition, detectors having high sensitivity are needed because the amplitude of the transmitted light is also quite low.
The most common and long-standing ultrafast gating methods used for imaging are the Kerr shutter and the streak camera. However, each has its own particular disadvantages in these applications. The Kerr shutter, although extremely fast (subpicosecond) and jitter free, requires a high peak power amplified laser system, which tends to be cumbersome and typically runs at low repetition rates, generally on the order of kHz. The Streak camera, on the other hand, while convenient to use, cannot attain subpicosecond jitter-free operation at high repetition rates. Typical resolution specifications for synchroscan-type cameras having high repetition rates are only on the order of 5-10 picoseconds.
Holographic gating in the form of light-in-flight (LIF) holography or chronocoherent imaging, has also been used to isolate the first-arriving light. These techniques provide jitterfree, low-power, high-pulse-repetition-rate, two-dimensional imagery, with a temporal resolution virtually the same as the pulse duration. However, the LIF-type configuration does not necessarily produce the best spatial resolution with such short pulse durations.
The limitations of the holographic methods are considerable. First, the portion of the light which is not coincident with the reference beam, and therefore does not interfere with it, nonetheless contributes to the exposure process, producing ambient background which lowers the contrast of the recorded interference pattern and subsequently raises the noise level. If the pulse is lengthened by, for example, a factor of 10.sup.4 by the scatter process, then the background light will be of the order of 10.sup.4 greater than the preferred light. This effect ranges from deleterious to disastrous, depending on the amount of scatter-induced pulse lengthening.
A second serious problem is that the exposure time available for recording the hologram is limited by the time over which the object motion is negligible. This is a basic problem of holography. If, for example, this stability time is 20 ms, a typical value for living tissue, the hologram exposure must be done within this time interval, otherwise the fringes to be recorded will be smeared. During 20 ms, many thousands of pulses might be recorded, but 20 ms is a rather short hologram recording time. If sufficient energy is to be delivered during this exposure time, the needed light intensity could be higher than can be tolerated.
Finally, conventional materials, such as photographic film, have low quantum efficiency, thus making extremely inefficient use of the light they receive. It is partly for this reason that electronic cameras have totally dominated over photographic film in such applications as computer-aided tomography.
Electronic holography is a form of holography wherein a hologram is formed on the surface of a detector, such as a CCD camera. The hologram is read out, typically into a computer, which then computes the image in a manner analogous to the conventional reconstruction process. The resulting image is displayed on a monitor. Since the resolution of a CCD camera is limited in comparison to photographic film, steps must be taken to insure that the signal falling on the detector is sufficiently coarse to fall within the spatial frequency capability of the camera. This form of holography, however, provides the advantage that the exposure time of each hologram can be quite short, and the holographic reconstruction process can be carried out digitally, and the image stored.
It is, therefore, an object of this invention to provide a system which simply and economically produces images of objects embedded in a diffusing medium.
It is another object of this invention to provide a system which produces images of internal features of a living being.
It is also an object of this invention to provide a method of propagating a light beam through the flesh of a living being to produce an image of the features therein.
It is a further object of this invention to provide a system for producing images of internal features of a living being without subjecting the living being to ionizing radiation.
It is additionally an object of this invention to provide a system for producing images of objects embedded in a diffusing medium, wherein the diffusing medium has a time-varying diffusion characteristic.